Polybutadienes are used as important constituents of rubber mixtures in the tyre industry, and it is desirable here to achieve an improvement in final properties, for example a reduction in rolling resistance and in abrasion value. Another application sector is provided by golf-ball cores or shoe soles, where high rebound resilience is a primary concern.
Polybutadienes having high content of cis-1,4 units have been produced on a large industrial scale for quite some time and are used for the production of tyres and of other rubber products, and also for impact-modification of polystyrene.
High content of cis-1,4 units is currently achieved almost exclusively by using catalysts based on compounds of the rare earth metals, as described by way of example in EP-A 1 0 011 184 and EP-B-A1 0 007 027.
It is known from the prior art that specifically neodymium-catalysed polybutadienes within the high-cis polybutadienes group have particularly advantageous properties in relation to rolling resistance, abrasion value and rebound resilience. The catalyst systems used play an important part in the production of polybutadienes.
By way of example, the neodymium catalyst used in industry is a Ziegler/Natta system composed of a plurality of catalyst components. Catalyst formation mostly involves formation of different catalyst centres, resulting in at least bimodal molar mass distribution in the polymer. The known 3 catalyst components in the Ziegler/Natta catalyst system are mostly composed of a neodymium source, a chloride source and an organoaluminium compound, these being mixed under particular temperature conditions in a very wide variety of ways, whereupon the catalyst system is prepared, with or without ageing, for the polymerization process.
The prior art discloses a plurality of production processes for Ziegler/Natta catalyst systems used for the production of polybutadienes.
Another document known from the prior art is EP 0 127 236, in which the catalyst is produced via mixing of neodymium oxides and neodymium alcoholates, and of carboxylates, with organometallic halides, and also with an organic compound at a temperature of from 20° C. to 25° C. It is also possible to mix the said 4 components at from 50° C. to 80° C. In this variant, the mixture is cooled to from 20 to 25° C., and then DIBAH is added. There is no mention of ageing.
EP 1 176 157 B1 discloses a process for the production of polybutadienes with reduced solution viscosity/Mooney viscosity ratio, in which catalyst production uses a preforming process. Here, the neodymium versatate is first mixed at 50° C. with DIBAH and isoprene, this mixture is then cooled to 5° C., and then ethylaluminium sesquichloride (EASC) is added. The ageing can take from a number of minutes to a number of days at a temperature of from 10° C. to −80° C. During the polymerization process, comonomers, for example a bisdiene, are added in order to increase the degree of branching of the polymer and thus also to obtain the very restricted solution viscosity/Mooney viscosity ratio. Because of the coupling by way of the bisdiene, the number of free chain ends per molecule in the resultant branched polymer is at least 4, whereas in linear molecules it is only 2.
The number of chain ends in the polymer is directly correlated with energy dissipation. As the number of free chain ends increases, energy dissipation through the polymer also increases. However, as energy dissipation in the polymer decreases, the rolling resistance of the polymer also decreases by way of example, and its rebound resilience improves. For identical molar mass, therefore, the final properties of a linear polymer having only 2 chain ends per molecule are accordingly always better than those of a branched polymer.
Commercially produced polymers are known to have a statistical molar mass distribution, and the breadth of the molar mass distribution here is influenced by the catalyst-production process.
The expression “step increase in Mooney viscosity” and similar expressions such as “step increase in Mooney value” or “Mooney jump” refer to techniques which significantly increase the Mooney viscosity of the polymers.
Capability to increase the molecular weight of elastomeric unsaturated diene polymers is important for various reasons. This permits initial production of low-molecular-weight parent polymers, with the great advantage, in the solution polymerization techniques usually used, of introducing lower viscosities in the “cement” (solution of the polymer in the organic-solvent medium used in the polymerization process), and of permitting operation with higher solids contents in the “cement”, since better heat transfer is achieved. It is also possible to reduce the cold flow of these diene polymers, thus increasing their capability for oil-extension.
It is well known from the prior art that use of solution polymerization methods for direct production of high-molecular-weight polymers, in particular high-molecular-weight neodymium-catalysed polybutadiene, is particularly difficult and uneconomic because of high solution viscosities. There are difficulties with stirring. Other phenomena are heterogeneity in the polymerization system and drastically reduced heat transfer. The direct polymerization process extending to high molecular weights would therefore necessitate lower rates of polymer production due to reduced solids content in the reaction space. This type of procedure increases the costs of polymer production considerably.
Although it is known a preforming process can alter the catalytic effect of Nd catalysts, and these preformed Nd catalysts give polymers with relatively low cold flow, the preforming process mostly reduces the activity of the catalyst, and neodymium consumption therefore sometimes rises considerably.
It is moreover known that polydienes with low cold flow can be produced if the diene polymers are treated after the polymerization process with disulphur dichloride, sulphur dichloride, thionyl chloride, disulphur dibromide or thionyl bromide (German Auslegeschrift 12 60 794). However, the method described in German Auslegeschrift 12 60 794 for the production of elastomeric diene polymers is that the said process is unsuitable for high-molecular-weight neodymium-catalysed polybutadiene if the step increase in Mooney value is to be at least 50% greater than the Mooney viscosity of the polymer after the polymerization process, because the “step-increase polymer” exhibits gelling, and this reduces reactor operating time, because of deposits on the internal walls of the reactor. The maintenance and cleaning of the reactors is time-consuming and expensive. There is moreover the risk that gel content is present in the actual polymer, which therefore cannot then be used for tyre applications.
DE 44 36 059 A1 likewise describes a method for achieving a step increase in the molecular weight of Nd-catalysed diene rubbers, where the intrinsic odour of the polymer is reduced via a depressurization step after the polymerization process, in order to remove all of the low-boiling-point constituents of the reaction mixture. The step increase in Mooney value here is about 27% greater than the Mooney viscosity of the diene rubber after the polymerization process.
The object of the present invention therefore consists in providing a simple, effective and economic process for achieving a step increase in Mooney viscosity, where the step increase in Mooney viscosity is at least 50% greater than the Mooney viscosity of the polymer after the polymerization process and at the same time exhibits no gelling or no significant gelling.